Bleaching chemical paper pulp



Patented Feb. 2, 1954 PATENT? OE-Fil BEEACHINGT CHEMICAU PAPER PULP FranciszlznEennelln Niagara Falls, N. Y., assignor to' sE. I. .-,du PontideaNemours and Company; Wilmington, DeL, .a corporation:,of -Delaware No Drawin'g. Application Septembcr22;"1950; SeflaLNo. 186,319 4 9 Claims. (Cl. 8-106):-

proved method-of bleaching chemical wood :pu lps; 2J2. A further. .object is .totprovidexa modificationrofi priollrmulti stage methods;zforxbleaching such pulpsginvolving a treatmentrwith-a.calcinmicoma: i pound; in .a stage. prior :to xthatzin avhichzthezapul e is treated: with :an ;a1kalinei.:peroxideisolutiongb which modification substantialbi;zbettembleach ing results: A particularcobiectiis to:provide:fan. improved multiesta'ge method. for :bleaching.;kraft.: 1. pulp in which an .alkalineixperoxide:ibleachin 1. stage fol-lows 0119301. more.xearlier.stagessintwhich;;? calciumhypochloritetis .useda; .stillzfiurthenaob. 3' cots will be apparent rfroxn .th erfollowingedescrim. tionis The present invention-is based onrthezdiscoverys f thateinathe:multiestageg bleachingziof zzchemical wood puil s-,;:substantiallm bettemhleachinanresults a; arezobt-ainedeif theapulniis aoidifiedifintermediat aneearlier-stageq -(orastages) in whiclrztherpulpwis treated-.-.-withw-a :calcium-rcompoundiiand \a sub quent-istage in:whichrl-it is treatectzwithmalkalineasg5 peroxide Thaimproyedzresults includetarhigher c. anctemorezstablerdegreeeof ,cbrightnessm-Furtherr more;-:-theeificiency qf thaalkaline.peroxidatreat ment:- is substantially improvedebmsuch inter. mediate,piaciclificatnmet Aceordingly thee aboll. -s objectsaare accomplished in accordancexwith 1th inventionabyacidifying:.ztheachemiealvpulp afterw it hascbeentsubjected'.toatreatment with a cal-.15 i m p nd, andrbefore-eit is ltreatedewithi anew.

alkaline...-peroxidesolution Acidification; of the pulp;mayebeeeffectede byintimately. contacting.-nthewpulpew thaanyaqueou solutioniof :an-\acid-in-suchv amounts-,andsconcen trationsithat .theiresultingwmixtura will .berdis tinctly acid, i. e., have a pga not greatemtham 6 Generally ,the,. pulp mill; begslurrieda with-thezacid solution but many ether: convenientqre or, desired method. cf \obtai ni-ngz intimate-woman .maye usedm Anyracidam.acid.,yielding gnaterialewhic will producethe.requiredaaciditu, maarbenused $86;

. 2 1'." The: invention :is further illustratedibyiathezfol-zs,- lowing examples.

Examplet'rl. I

The pulp samples used in. Examplesd to '4 .1 wer'e;taken from various stages of a millbleach-i. in process. in which 'kraft. pulpjrom. northernly; mixed hardwoods. is gbleache'cl in conventional; manner initwo'separate stages, employing 'ineachj'i. about 2% of active'chlorine (based on the dry}; weight'of the pulp) in'the"form.of calciumhypog. chlorite at a pulp consistency of about v112%.? The pulp is washed with water after eachbleach' ing stage. Washed pulpifromrzthe first stage had 15 an airbrightness .of.fi4.5, anoLan .ovenbrightness of 757.5. Washed pulpnfrdmlthe eseconistage :-v had airand .oven .bright'nesses of...75.0. .anda6745;, respectively; v

(a) Washed. pulp from .thelfirsit stag-e ofathem. above mill process. wast-bleached at. a pulpficona-w sistency of 12% with an alkaline peroxide bleach? solution containing 0.05% Epsom salts,.5.0% 41.6-9- Bl sodium silicate (containing 8.9%..Na20,-.j29.% 3; S102" and the balance water) .-1.33 sulturic acid; and 2.0 sodiumlperoxide, allbasedaoni the dry; a weight of the. pu1p..1, A .beakercontainins a slurry. a of the pulpfand bleach. .liqjuor wasa-placedein .ane oven." The" temperature. ofethe slurry..roseain.i2 hours: from Jit0 "F. f Since .at thishpoint.-thea r residual active oxygen content...was. only; 19% of 'that. originally'iadded, bleaching -.was..discon-+ tinuedand the air andoven.brightnesses..,were iN foundto'be 71.0 ancl 67.0, resp.ectively-. ..-.The of the slurry duringthe' peroxide-"bleach: rangedm from"'about 10:5 to 10.8...

(b)iA second sample (of washedtpulp from :the firs't sta'gepf the mill "process ,was. .slurried-xatW about 2% consistency Withwatenicontaining.hyzdrochloric" acid in an amountv equal to 3.0% fhy fyr drolch-loric acid of about 35%"strengthwbasedion thedry Weight of .the' pulp.1' 'Ihresultihaslurny hada pH ranging'from about-2.01to.2.6ifAfte1:e.- about lfl minutes, the" pulp was washed than-i; oughly withwater thensubj'ected to .an alkalineta. peroxide bleach. under the same conditionswas e.,

in (a)-abo've: Theresidual activeioxygenlcomm I nesses of thepulp were 79.5 and. 7.7.5; respectively of pulp and peroxide bleach liquor had risen to 160 F. Because the residual active oxygen content then was only 12%, bleaching was discontinued. The air and oven brightnesses of the peroxide bleached pulp were 74.5 and 69.5 respectively.

Example 1 (b) was repeated using pulp from the second stage of the mill process. After 5 hours in the oven, the temperature was 180 F. and the residual active oxygen content was 44%. The final air and oven brightnesses of the pulp were 82.0 and 79.5, respectively.

Emample 3 A sample of washed pulp from stage one of the above mill process was acidified as in Example l (b) except that 2.5% hydrochloric acid was used. The pH of the acidic slurry ranged from about 2.6 to 3.0. The acidified pul was washed, then bleached with alkaline peroxide as in- Example '1 (19) except that no sulfuric acid was added to the bleach liquor, the silicate and sodium peroxide contents were reduced to 2.0% and 1.0%, respectively, and the slurry of the pulp in the bleach liquor was heated on a hot plate instead of in an oven for 1 hour. During this time the temperature rose to 200 F. and the pH ranged from about 11 to 11.3. The residual active oxygen content at the end of the treatment was 5%. The air and oven brightnesses of the bleached pulp were 76.0 and 74.5, respectively.

Example 4 Example 3 was repeated using washed pulp from the second stage of the mill process. Here again, the final temperature during the peroxide bleach was 200 F. and the residual active oxygen content after 1 hour was 5%. The final air and oven brightnesses were 80.0 and 78.0, respectively.

In the receding examples, brightnesses were determined using handsheets prepared from pulp slurries acidified with dilute sulfuric acid to'a pH of 4.5 at 03-05% consistency. Brightnesses were measured using a General Electric Refiectometer, which measures brightness on a scale ranging from 0 for minimum reflectance to 100 for maximum reflectance. Air brightnesses; were determined using handsheets dried at atmospheric temperature. Oven brightnesses were determined after drying freshly prepared handsheets for 1 hour in an oven heated'to 105 C. (221 R). The smaller the difference between the air and oven values for a given sample, the greater is the stability of the brightness of that sample.

The results of Examples 1 and 2 show that the final peroxide bleach is much more efiective if preceded by acidification. They also show that preacidification substantially and eifectively decreases peroxide decomposition in the final bleach; These observations hold .for pul from either the first or second hypochlorite stage of the mill process. However, it will be noted from Example 2 that whereas the brightness of pulp from the second stage was increased only slightly by the peroxide bleach in the absence of preacidification, the brightness increase from the final peroxide bleach was very substantial when the pul was preacidified. These results accord with general experience in applying the present invention, i. e., that the value of the acidification treatment increases the more the pulp has been contacted previously with calcium Compounds- Thus, the invention is most advantageously applied to pulp which has been previously subjected to a plurality of calcium hypochlorite bleaching stages, or previously contacted in several stages with calcium compounds, e. g., with calcium containing liquors recycled from various stages of commercial processes. Repeated contact with calcium-containing liquors appears to be particularly harmful to bleaching in a subsequent alkaline peroxide bleach, unless the latter is preceded by the present acid treatment.

It will be noted that 1% peroxide with preacidification gave much higher brightnesses in Examples 3 and 4, than did twice that amount of peroxide without preacidification in Examples 1 (a) and 2 (a). Also, 1% peroxide with preacidification gave substantially higher brightnesses starting with pulp from stage 1 of the mill process (Example 3), than did 2% peroxide without preacidification when starting with pulp from stage 2 (Example 2 ((1)).

Example 5 A kraft pul composed of about northern softwood (spruce and fir) and about 20% northern hardwood (poplar), was used in this example after it had been partially bleached by a multi-stage method in a commercial pulp mill. The partial bleach involved in the following order: (1) a chlorination with about 5% chlorine at about 3 to 3.5% consistency; (2) a second chlorination at about the same consistency with about 2.5% chlorine, in which the pul is alkalized after a few minutes with lime to a pH of about 9 to 11; (3) a caustic extraction using 2% caustic at about 12% consistency and 180 F.; (4) an alkaline soak; and (5) a calcium hypochlorite bleach equivalent to about 1.5% chlorine for about 1 hour at F. and 12% consistency. In all of the above treatments, the stated amounts of reagents are based on the dry weight of thepulp. Also, the pulp was Washed following each step. Samples of pulp pretreated in the mill in this manner had air brightnesses ranging from about 78-79 and oven brightnesses ranging from about 69-71. (In this case and in all subsequent examples, the oven brightness was determined for handsheets which had been dried for 1 hour at 0. (248 F.)).

(a) A washed sample of pulp partially bleached in the mill as indicated above was slurried at about 2% consistency with water containing hydrochloric acid in an amount sufiicient to give the slurry a pH of 2.0. After about 10 minutes,

, the pulp was washed free of acid then bleached at 12% consistency at F. and a pH of 11.3 with a peroxide solution containing 0.025% Epsom salts, 1% 41.6" B. sodium silicate solution and 1% sodium peroxide. After 1 hour, the residual active oxygen was 73% and the air and oven brightnesses were 84.3 and 81.1 respectively.

After 3 hours, the residual active oxygen content was 60% and the air and oven brightnesses were 86.3 and 82.0, respectively.

(b) Example 5 (a) was repeated, omitting the acid pre-treatment. After only 1.5 hours the residual active oxygen content had dropped to 10%, in view of which bleaching was discontinued. The air and oven brightnesses were then 83.0 and 77.7, respectively.

(0) Example 5 (b) was repeated except that 2% each of silicate and sodium peroxide were used in the bleach liquor (pH 11.3) The residual active oxygen after 1 hour bleaching was only 12% and the air and oven brightnesses were 85.0

azecs ocs .an 7. 3.,re spective1y. illhe oven bri htness in this instance qu i-ns twice, theramountpf peroxide was substantiallyslower-than the value obtained 1m thesameatime iii-(Example a5 (a?) -.in-..which the pulp was preacidified.

Ewample 6 .Pulp from thesource described inwExamplefi Four washedpulp samples from the "sourcedescribed in Example -5 were bleached at 150 F., a

consistency of 12% and a'pH of 11.3, 'using a bleach liquor containing '-0.-012% Epsom salts, 1i0-% 4ls6 B'. sodium silicate and 1:0% sodium peroxide. Sample 1 was bleachedwithout acid pretreatment. Theremaining samples were pretreated with hydrochloric acid in the manner described in Example 5. The amounts of hydrochloric acid used, based on the weight of the .pulp were: 1.0% for sample 2, giving .a pH of 1.8; 1.25% for sample 3, giving a pH of. 3.-5; and, 2.5%,for sample 4, giving a pH of 2.5,. 'After acidifying, samples 2.3 and 4 were washed free of .acid then bleached as indicated; The residual active oxygen contents after 1.5 hours fcrsamples 1 to 4 were 11%, 22%, 35% .and 46% respectively.

Final oven brightnesses'for samples 1 to 4 were 5 716.7, "19.4, 81.1 and 80.9, respectively.

Example 8 (b) Example 8 (a) was repeated except that chlorine was substituted for hydrochloric acid as the acidifying agent in the pretreatment step. 3% C12, based on the weight of the pulp, was required to give a pH of 3.3. After a peroxide bleach time of 1.5 hours, the residual active oxygen content was 40 and the air and oven brightnesses were 88.3 and 83.2, respectively.

The results of Examples 8 (a) and (b) show that chlorine can be used in place of hydrochloric acid in the acid pretreatment step with substantially identical results. In other experiments substantially the same as those of Examples 3 (a) and (b), except that the peroxide bleach step was omitted, it was found that the acidifying treatment, using either hydrochloric acid or chlorine, did not itself result in any substantial increase in brightness, the maximum increase being at best about 1 point.

While the foregoing examples illustrate applications of the invention in bleaching kraft pulps, the invention can also be applied advantageously in the bleaching of other chemical pulps such as soda pulp, sulfite pulp and semi-chemical pulps. In the case of any of these chemical pulps, the acid treatment of the invention should follow one iii) -1or .more. preliminarystreatments which .8;..081- .oiumacompound, such- :as 1 11119101 :calciumthypochlorite, is-:-used, .-.-and. will "of icourse *zprecede a subsequent nalkal-ine peroxidebleach. This :acid treatmentvis; how-ever,- jeparticularlywell suited for use :inibleachingkralit -(-sjulfate) "pulpsavhich are normally :darker in color 'and are moredif- .ficult :to-rblea'ch to a high permanentbrightness. Kraft 'gpulps' may be bleached 'to' excellent :and moreipermanent'brightnesses by t'he present improved process than 'is possible "with corresponding prior combination calcium .hypochlorite peroxide processes. Furthermore, the same -degree -of-brightness=may='be 'bbta'ine'd by the present eprocess, asrcomparedito corresponding :prior proc- 'esses,v employing .smaller amounts of peroxide or ica'lcium hypoc'hlorit'e; thus permitting a substantial ireduc'tion' in costs. Reduction in the amount .of hypochlorite used is known to reduce pulp "degradation.

fDh'e invention i'srsnot restricted to processes employing any specific number or types of steps except that the acid treatment must be intermediate a treatment witha calcium compound and a later treatment with alkaline peroxide. The icommon practices of first partially purifying the apulp by prechlorination treatments, by alkali treatments, or combinations thereof, are includ ed, as are also combination partial bleaches using both sodium andcalcium 'hypoc-hlorites. Also, the alkaline peroxide bleach after the present acid treatment, may be supplementedby subsequent treatments, e. "g;, with hypochlorite, perox-ide and/or other treating-"agents, including reducing agents.

' As previously indicated any acid or acid yielding substancemaybe used in the acidification step which will give therequired acidity. To be efiective, the acidity should correspond to a pH no -higher than'fi. In most applications, a'cidities corresponding to a'p'H of 'about'2 to 5, preferably 2.5 to 4.0, will be used. Acidities greater than a pH-of about 2 offer'no particular advantages and increase 'costs. Illustrative o'fsu'itab'le acidifying agents are -=i h ydrochloric,-"nitric and -phosphoric acids, and chlorine." Acids w'hichgive the more soluble calcium salts, such ashydrochloric acid, and chlorine are preferred. These will ordinarily and most conveniently be applied as aqueous solutions at low pulp consistencies, e. g., 0.5 to 5%, and at ordinary temperatures. However, pulp consistency and temperatures appear not to be critical with respect to the acid treatment and high or low consistencies and temperatures substantially higher or lower than ordniary may be used if desired. When using chlorine, temperatures above about 100 F. are generally avoided, temperatures below about F. being preferred. Under these conditions the chlorine functions primarily as an acidifying agent and does not oxidize or degrade the pulp to any substantial extent. Prolonged contact of the acid with the pulp is not necessary, 5 to about 15 minutes being generally suitable. Longer times may, however, be used.

The bleaching and other steps of the prior processes to which the present acidification step is applicable may be carried out in any desired manner according to well-known procedures. Also, as is Well known, the alkaline peroxide bleach which follows the acidification step may be practiced using any peroxygen compound which yields hydrogen peroxide in aqueous alkaline solutions, examples of which are hydrogen peroxide, percarbonates, perborates, perphosphates'and alkali'metal peroxides. Because of its relative cheapness, sodium peroxide is preferred, although a combination of hydrogen peroxide and an alkali may be used with equally good results. Ordinarily, the alkaline peroxide bleach will be effected at a pH of 9.5 to 13, preferably 10 to 12; and at temperatures of MiG-200 R, preferably 150-190 F. Higher or lower alkalinities and temperatures may also be used. The various bleaching stages may be carried out at any pulp consistencies customary or desirable for any given process.

I claim:

1. In a process for bleaching chemical wood pulps wherein the pulp is treated under alkaline conditions with a calcium compound yielding calcium ions and then treated with alkaline peroxide, the improvement comprising acidifying said pulp intermediate said treatments, without simultaneously bleaching saidpulp to any substantial extent, with an agent from the group consisting of hydrochloric acid and chlorine to a pH not higher than 6.

2. In a process for bleaching chemical wood pulps wherein the pulp is treated with calcium hypochlorite and then treated with alkaline peroxide, the improvement comprising acidifying said pulp intermediate said treatments, without simultaneously bleaching said pulp to any substantial extent, with an agent from the group consisting of hydrochloric acid and chlorine to a pH not higher than 6.

3. In a process for bleaching chemical wood pulps wherein the pulp is treated under alkaline conditions with a calcium compound yielding calcium ions and then treated with alkaline peroxide, the improvement comprising acidifying the pulp intermediate said treatments, without simultaneously bleaching said pulp to any substantial extent, with an agent from the group consisting of hydrochloric acid and chlorine to a pH of 2 to 5.

4. In a process for bleaching kraft pulp wherein the pulp is treated under alkaline conditions with a calcium compound yielding calcium ions then treated with alkaline peroxide, the improvement comprising acidifying said pulp intermediate said treatments, without simul taneously bleaching said pulp to any substantial extent, with an agent from the group consisting of hydrochloric acid and chlorine to a pH of 2 to 5.

5. A process for bleaching kraft pulp comprising partially bleaching said pulp with calcium hypochlorite, acidifying the partially bleached pulp, without simultaneously bleaching said partially bleached pulp to any substantial extent, with an agent from the group consisting of hydrochloric acid and chlorine to a pH of 2 to 5 and further bleaching the pulp with an alkaline peroxide solution.

6. The process of claim 5 wherein the pulp is acidified to a pH of 2.5 to 4.

7. The process of claim 5 wherein the alkaline peroxide bleach is carried out at a temperature of to F. and at a pH of 10 to 12.

8. The process of claim 5 wherein the acidifying agent is hydrochloric acid.

9. The process of claim 5 wherein the acidifying agent is chlorine.

FRANCIS L. FENNELL.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,768,823 Bradley July 1, 1930 1,771,06 Rue July 20, 1931 1,827,857 Rue Oct. 20, 1931 2,026,068 Scheller Dec. 31, 1935 2,083,280 Sunesson June 8, 1937 2,140,863 Sunesson Dec. 20, 1938 2,203,212 Casciani June 4, 1940 2,494,542 Casciani Jan. 17, 1950 FOREIGN PATENTS Number Country Date 573,341 Germany Mar. 31, 1933 OTHER REFERENCES Aitken: Peroxides to Bleach Sulphate Pulp. Canadian Chemical Processing, February 1953. pages 42 and 44. 

1. IN A PROCESS FOR BLEACHING CHEMICAL WOOD PULPS WHEREIN THE PULP IS TREATED UNDER ALKALINE CONDITIONS WITH A CALCIUM COMPOUND YIELDING CALCIUM IONS AND THEN TREATED WITH ALKALINE PEROXIDE, THE IMPROVEMENT COMPRISING ACIDIFYING SAID PULP INTERMEDIATE SAID TREATMENTS, WITHOUT SIMULTANEOUSLY BLEACHING SAID PULP TO ANY SUBSTANTIAL EXTENT, WITH AN AGENT FROM THE GROUP CONSISTING OF HYDROCHLORIC ACID AND CHLORINE TO A PH NOT HIGHER THAN
 6. 